The History of X-ray Astronomy

X-ray astronomy is a relatively new science as we have needed
satellites to do it; these have only been around since the 1950's.
X-rays have a tendency to go through things compared to visible light,
but they are also fairly easily absorbed - a few millimetres of bone
or a few metres of air will stop them. This is good for us, as large
doses of X-rays are dangerous and can cause cell mutations, and there
is plenty of air between us and space. So we are lucky that the Earths' atmosphere
blocks X-rays from reaching the Earths' surface. This is because the
atmosphere contains water which is opaque to X-rays. If all the water
were in the atmosphere were condensed onto the surface, it would form
a layer 10 m deep. However this does
mean that we need to get into orbit to look at X-rays from space. The
picture shows how different wavelengths of electromagnetic radiation
("light") are absorbed by the atmosphere. Therefore we need satellites to be able to do any X-ray astronomy.

The
depth of
penetration of different frequencies of light into the Earths
atmosphere. This shows why we need satellites for X-ray astronomy. (Image courtesy NASA)

Initial Missions

The first attempts at X-ray astronomy were just to see if there
was any form of X-ray radiation in space. The instruments were simple
detectors carried aloft on rockets which then parachuted back down to
Earth. They detected X-ray emission from
the Sun. The surface of the Sun is relatively cool (5800K) and so doesn't
produce many X-rays, but is therefore a very good emitter of visible
light. For an object to emit most of its light in X-rays its
temperature has to be ~6,000,000 degrees. However the Sun is surrounded by a corona which is a much
stronger emitter of X-rays as it is super-heated to ~1,000,000K. Modern images of
the Sun in X-rays using the Japanese
Yohkoh satellite are shown below.

The
extended Corona of the Sun as seen in X-ray by the Yohkoh Satellite. (Image courtesy Yohkoh)

Subsequent to this success more missions were sent up to try and
detect reflected X-rays from nuclear weapons tests. From the emission from the Sun
it had been determined that there would be no detectable emission from
any normal stars so it was not worth looking elsewhere.

Then in 1962 a simple X-ray detecting payload went up from New
Mexico to try and detect reflected X-ray emission from the Moon. The
rocket was above 80 km for 5 minutes and 50 seconds and reached a
maximum height above the Earth's surface of 225 km. The payload
rotated in space and it was expected that there would be peaks in the
X-ray emission as it pointed at the Sun and the Moon. These were
detected, but there was something else there that was much brighter,
coming from the constellation of Scorpius and was called Sco X-1. It
is the brightest object in the X-ray sky, much brighter than the full
Moon. Suddenly there was the possibility that there were things in
space that could be researched with X-rays. Giacconi won the Nobel
Prize for Physics in 2002 for his development of X-ray astronomy.

This mission also discovered an X-ray background - a hard X-ray
sky-glow which peaks at the galactic poles. This comes from the
"Local Bubble" which surrounds the Solar System. There is an
Extra-galactic X-ray Background which has been a topic of active
research and which is thought to be a result from sources which have
not been resolved with current X-ray telescopes.

The X-ray Sky

The
Constellation of Orion as seen in Optical Light.

The same region as seen in X-rays.

The image on the left shows the constellation of Orion in with the
Moon to the top. On the right is the same region of the sky as imaged
in X-rays. There are still many objects in the image all giving off
X-rays. You can see the Moon if you know where to look - it does
reflect some X-rays from the Sun (see section on the X-ray Background
of a clearer image). You can also see Sirius as a bright X-ray
source; however it is not the normal bright star (Sirius A) which you
see in the X-ray image, it is its companion, the much smaller and
hotter White Dwarf Sirius B. It was known that Sirius A had a
companion that was faint in visible light as its passage across the
sky wobbled. However it was not until much later that the companion
was imaged. It is the remnant of a much larger star that came to the
end of its life and died by throwing off its atmosphere and leaving
its naked, hot core to cool into space - it is hot enough to emit
X-rays.

The bright blue source to the top of the X-ray image is the Crab
Pulsar - another dead star, but this is what is left of one which died
in a much more spectacular fashion. The star exploded when it ran out
of fuel, and the core collapsed into a neutron star - a gigantic
atomic nucleus about 3 km across. This is a very hot star which
causes the gas cloud created as the star died to glow as a nebula -
the Crab Nebula.

Solar System Sources of X-rays

Although most X-rays come from outside of the Solar System, there
are some sources in our very local Universe as well. We have already
seen that the Sun is a source of X-rays and it causes most of the
X-rays seen in the Solar System. The Aurorae on Earth are sources of
X-rays, as are those on Jupiter and Saturn. The Moon reflects some of
the Solar X-rays which are detectable. The image
below shows the difference between the sunlit and dark sides of the
Moon and also the fact that there is an X-ray background on the sky.
Mars has been detected in X-rays for the same reason.